Non-plasma halogenated gas flow prevent metal residues

ABSTRACT

An apparatus and process for limiting residue remaining after the etching of metal in a semiconductor manufacturing process, such as etching back a tungsten layer to form tungsten plugs, by passivating the surface of a wafer with a halogen-containing gas are disclosed. The wafer is exposed to the halogen-containing gas in a chamber before a metal layer is deposited on the wafer. The exposure can occur in the same chamber as the metal deposition, or a different chamber. The wafer can remain in the chamber or be moved to another chamber for etching after exposure and deposition.

CROSS REFERENCE TO A RELATED APPLICATION

This application is a division of 08/942,582, filed Oct. 2, 1997, whichis a continuation in part of the application Ser. No. 08/625,485entitled “Non-Plasma Halogenated Gas Plow to Prevent Metal Residues” bySteve G. Ghanayem, et al., filed Mar. 29, 1996, now U.S. Pat. No.5,709,772 which is herein incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to metal deposition on semiconductorsubstrates in semiconductor processing systems, and in particular to theelimination of residue remaining after the etching of the metal.

One of the steps in the formation of a semiconductor chip is thecreation of metal interconnections between devices on a semiconductorwafer. Typically, this is done by first depositing a layer of metal,such as tungsten, across the wafer. In one process, tungsten isdeposited in a chemical vapor deposition (CVD) chamber by reacting WF₆containing the tungsten and H₂ or Silane at elevated temperatures over awafer sitting on a resistive heater (alternately, other methods may beused to heat the wafer, such as lamps). The process temperature istypically 475° C. Subsequently, the tungsten is etched away except forareas in which metal interconnections are desired. This may be done inthe same chamber or by moving the wafer to a separate chamber.

It has been discovered that under certain processing conditions, whenthe wafer is later etched, tungsten residue remains on the wafer, whichcan form shorts in the interconnects. Although the process is notcompletely understood, it is believed that tungsten may preferentiallygrow in certain spots on the wafer (These spots may be areas whereresidue has formed as a result of previous processing steps). Thepreferentially growing tungsten may be in a different phase than otherareas of the metal tungsten layer. Because the tungsten is in adifferent phase, these areas may then etch away at a different rate,leaving the undesirable tungsten residue in areas where there should beno interconnect after the etch step.

One approach to eliminate the residue is to clean the chamber in whichthe metal layer is deposited after every wafer is processed. This istypically done by removing the wafer, and then igniting a plasmacomposed of NF₃, followed by a hydrogen plasma, with a subsequent argongas purge. After this cleaning step, the next wafer can be inserted intothe chamber for depositing of the tungsten layer without significantresidue being present after etchback. A drawback of this plasma cleanapproach is that it requires a significant amount of time between eachwafer being processed. One approach would be to do the plasma lessoften. But it has been discovered, for example, that a plasma cleanafter every 25 wafers is not sufficient since residue formation stilloccurs.

Accordingly, it would be desirable to have an improved process foreliminating residue formation after tungsten deposition and etchbackwhich does not significantly impact wafer throughput.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and process for limitingresidue remaining after the etching of metal in a semiconductormanufacturing process by exposing a wafer to a halogen-containing gaswithout a plasma in a processing chamber. The wafer may be exposed tothe remnants of a halogen-containing gas remaining in the chamber frombefore the wafer was loaded into the chamber, or halogen-containing gasmay be injected onto the wafer before deposition of a metal layer. Theexposure can occur in the same chamber as the metal deposition, or adifferent chamber. The wafer can remain in the chamber or be moved toanother chamber for etching after exposure and deposition.

In one embodiment, the halogen-containing gas is NF₃ and is injectedinto the chamber between wafer processing steps, although other gases,such as C₂F₆, CF₄, ClF₃, or Cl₂ may be used. Preferably, thehalogen-containing gas is injected for less than 30 seconds and at arate of between 10-2000 sccm (more preferably between 10-150 sccm) witha pressure between 50 milliTorr (mT) and 90 Torr (T) (more preferably,between 50 mT and 10 T), but the halogen-containing gas may be injectedfor up to 150 seconds. A longer injection period would be appropriatewith a batch reactor, for example. In general, it is desired that theinjection period be short, relative to the total clean time.

In another embodiment, a halogen-containing gas is injected onto thewafer after the wafer has been loaded into the chamber. Thehalogen-containing gas is WF₆, and serves as a fluorine source thatprevents metal residue after a subsequent deposition and etch step.

In yet another embodiment, a plasma chamber cleaning operation isperformed after a number of deposition steps. The plasma is formed froma halogen-containing gas, for example, NF₃. The plasma is maintained to“overetch” the chamber, that is, the plasma is maintained beyond thetime required to clean unwanted residue from the chamber. It is believedthat the plasma provides a source of fluorine during the overetch step,and that the fluorine is adsorbed onto the walls of the chamber. Thewalls of the chamber subsequently provide fluorine to passivate thesurface of wafers during metal depositions.

For a further understanding of the nature and advantages of theinvention, reference should be made to the ensuing description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a semiconductor wafer processing system used inone embodiment of the present invention;

FIG. 2A is a flowchart of a process according to an embodiment of thepresent invention using an overetch technique;

FIG. 2B is a flowchart of a process according to an embodiment of thepresent invention using an indirect exposure method;

FIG. 2C is a more detailed version of the flowchart of FIG. 2B; and

FIG. 2D is a flowchart of a process according to an embodiment of thepresent invention using a direct exposure method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One suitable system for carrying out the present invention is shown inFIG. 1, which is a diagram of a semiconductor substrate processingsystem 10 including a cross-sectional view, partially schematic, of aCVD chamber 12. A substrate 14 is shown sitting on a heater mount 16.The substrate is brought into chamber 12 by a robot blade through a slitvalve in the sidewall of the chamber (not shown). Chamber 12 may be partof a vacuum processing system having a plurality of processing chambersconnected to a central transfer chamber. The heater mount 16 is movablevertically using a motor 18. The substrate 14 is brought into thechamber when the heater mount 16 is in a first position opposite theslit valve. The substrate 14 is supported initially by a set of pins 20that pass through the heater mount 16. When the heater mount 16 israised to a processing position, the wafer is lifted off of the pins. Agas distribution plate 22 provides process gases into chamber 12. Gasesare provided from a gas supply and mixing system 24. An exhaust system26 evacuates the reaction byproducts and unreacted gases.

Wafer 14 is heated during the process by a resistive coil 28 embedded inthe heater mount 16. Coil 28 exits the heater mount 16 at the bottom ofFIG. 1 and is connected to an external power supply (not shown). Thebottom of FIG. 1 also shows a thermocouple connection 30 for measuringthe temperature of the heater mount 16. Also shown is a vacuum line 32which connects to recesses (not shown) on the top of heater mount 16 forholding wafer 14 in place.

A pair of purge lines, 34 and 36 are shown for providing purge gases.Purge gas line 34 provides a purge gas which comes up around the edgesof heater mount 16 and past purge guide 38, which will form a small gapbetween it and wafer 14 in the processing position. This purge gasprevents any tungsten from depositing on the backside or edges of wafer14. Additional purge line 36 is used to keep any residue from collectingaround stainless steel bellows 40 which provide a barrier between themechanical mechanisms and the chamber interior.

The process performed in chamber 12 can either be a CVD process, or aplasma enhanced CVD (PECVD) process. In a PECVD process, RF power isapplied between gas distribution plate 22 and heater mount 16 by an RFpower supply 42. Typically, heater mount 16 is grounded, with thepositive voltage being applied by a strap to a support plate for gasdistribution plate 22. RF power supply 42 can supply either single ormixed RF power to enhance the decomposition of reactive speciesintroduced into chamber 12.

A processor 44 controls the operation of the chamber, in particular theRF power supply and the gas supply and mixing system 24. The processoruses a memory 46 which stores a program containing instructions foroperation of the system. In addition, the processor can control a gasinjection source through gas mixing system 24. The processor alsocontrols the purge lines, a vacuum pump connected to the vacuum linesfor holding the wafer, and the vacuum exhaust system. The processor alsoreceives a signal from the thermocouple 30 to provide feedback forcontrol of the power supply connected to coil 28 for providing thedesired temperature through resistive heating. Additionally, processor44 can control motor 18 for moving the heater mount 16 as desired, aswell as the robot arm (not shown).

It is desirable to process as many wafers as possible during a givenperiod of time. However, as discussed above, periodic chamber cleans aretypically performed to remove deposits on the interior of the chamber.Chamber cleans are not done after each deposition for at least tworeasons. First, a typical clean involves striking a plasma in thechamber between wafer depositions and waiting until the unwanteddeposits are removed from the surfaces of the chamber. Such a cleaningstep takes time away from wafer production. Second, a plasma cleanchanges the environment of the chamber.

One type of periodic chamber clean is a plasma clean using a halogenatedprecursor gas, such as NF₃. The plasma dissociates the NF₃ into reactiveplasma species, including fluorine ions and fluorine free radicals.During the chamber cleaning process, the reactive plasma species combinewith the unwanted deposits to form compounds that are removed by thechamber exhaust system. Various methods may be used to determine theendpoint of the cleaning process, that is, when all of the unwanteddeposits have been removed. One such method measures the spectralemissions of the glow discharge of the plasma, and ends the cleaningprocess when the spectra corresponding to emissions from elementspresent in the deposits drop below a selected level.

Generally, it is desirable to stop the inter-wafer plasma cleaningprocess after the unwanted deposits have been removed. However, anunexpected benefit has been observed when the chamber is “overetched.”Overetching a chamber involves maintaining a plasma from the halogenatedprecursor for a longer period of time than is necessary to remove theunwanted deposits from the chamber. Overetching a chamber providesbenefits similar to treating a wafer with a non-plasma halogenated gasprior to deposition. Specifically, overetching a chamber reducesunwanted residue on the surface of wafers processed after theoveretching has occurred. It is believed that free fluorine liberatedduring the overetch period of the chamber plasma clean process adsorbson the chamber surfaces, and that this adsorbed fluorine subsequentlypassivates the surface areas of wafers where a residue might otherwiseoccur.

The beneficial effects of the overetch period provided superior results,i.e., reduced residue, for a number of wafers that were processed withdeposition and etch steps subsequent to the overetch clean. In aspecific example, a processing chamber was overetched for a period of180 seconds with a plasma formed from NF₃, but it is believed that thedesired effect would be obtained for overetch periods as short as 15seconds and as long as 300 seconds. Subsequently, wafers were processedin that chamber by first forming a 7000 Å thick layer of tungsten fromWF₆ and H₂ using a thermal CVD process. The wafers were then etched backto form tungsten plugs in pre-existing interconnection vias in thewafer. The beneficial effects of overetching the chamber were seen for25 wafers processed after the overetch process. It is believed that thisnumber of wafers can be increased, to a limit, by increasing the plasmadensity during the overetch period and/or increasing the overetchperiod. Therefore, it appears that increasing the overetch periodincreases the amount of fluorine adsorbed onto the chamber surfaces thatis subsequently available for wafer surface passivation.

The beneficial effects of passivating a surface with ahalogen-containing gas were seen subsequent to a “blanket” etchback, asdescribed above in connection with a tungsten-plug process. It isexpected that a similar benefit would be obtained when etching apatterned layer of metal. In a patterned etchback, the wafer would bepassivated with a halogen-containing gas prior to depositing a layer ofmetal, such as tungsten. Then, the wafer would be removed from thechamber to a lithography line where, typically, a resist would beapplied to the wafer, exposed, and developed to form a resist pattern onthe metal layer. The wafer with the patterned resist layer would then beetched, as by placing the wafer in an etch chamber. Passivating thewafer prior to deposition of the metal layer would reduce the residueremaining after the etch step. In a blanket etchback process, the wafercould be etched-back in the same chamber the metal was deposited in, butis typically moved to another chamber in a multi-chamber processingsystem, or moved to another chamber in another system.

FIG. 2A is a simplified flowchart illustrating one embodiment of thepresent invention used in the chamber of FIG. 1. After a wafer is loadedinto the chamber (step 201) it is processed by depositing a metal layeronto the wafer (step 203). The wafer is then removed from the chamber(step 204) and a determination is made whether a chamber plasma clean isdue (step 205). This determination may be made based upon the number ofwafers processed, for example, after 25 wafers, or may be based upon thetotal thickness of metal deposited since the previous chamber clean, orother parameters. If a chamber clean is not due, another wafer is loadedand processed. If a chamber clean is due, cleaning precursor gas isinjected into the chamber (step 207) and a plasma is struck (step 209).The plasma is maintained for a time period sufficient to remove thedeposits from the chamber (step 211). In this instance, the plasma ismaintained for 600 seconds at a chamber pressure of 350 mTorr and aplasma energy density of 1.9 W per cm².

A determination is made whether more wafers are to be processed (step213). If no more wafers are to be processed, the process is stopped. Ifadditional wafers are to be processed, the plasma is maintained for anoveretch period (step 215) of 30-180 seconds. It is believed thathalogen species from the plasma become adsorbed onto the surfaces of thechamber. After the chamber is overetched, the plasma is extinguished(step 217) before loading another wafer and proceeding. A reduction inresidue remaining after a metal etch process was observed on 25 wafersthat were processed in a chamber following the above overetch process.

Another way to passivate the surface of a wafer with a halogen to reduceunwanted residue from a subsequent deposition and etch process is toexpose the wafer to a halogenated gas prior to the deposition step. Thisexposure may be indirect, as from remnants of a halogen-containing gasremaining in the chamber after a chamber cleaning process or injectedinto the chamber at the end of a deposition process, or may be direct,such as flowing non-plasma halogenated gas directly on the surface ofthe wafer, prior to initiating deposition.

FIG. 2B is a flowchart illustrating one embodiment of the presentinvention used in the chamber of FIG. 1. It should be understood thatother chambers or systems could be used to implement the presentinvention. First, halogen-containing gas is flowed to the chamber by acommand from the processor 44 to the appropriate valves in gas supplyand mixing system 24 (step 219). Subsequently, a wafer is inserted intothe chamber by a robot arm under control of the processor (step 221).Next, a layer of metal, preferably tungsten, is deposited on the wafer(step 223). Next, the wafer is etched, either in the same chamber or ina different chamber or different system (step 225). Alternatively oradditionally, a halogen-containing gas may be injected into the chamberafter the metal layer is deposited on the wafer, thus providing remnantsof halogen-containing gas in the chamber to passivate the surface of asubsequent wafer.

FIG. 2C is a more detailed flowchart of the embodiment illustrated inFIG. 2B. After depositing the metal on the previous wafer in the chamber(step 227), the previous wafer is removed (step 229). NF₃ is then flowedat a flow rate of approximately 30 sccm by control of a valve in the gassupply and mixing system by the processor (step 231). Alternately, flowrates between 10 and 2000 sccm or more preferably between 10 and 150sccm may be used, as controlled by an instruction in memory 46 executedby processor 44.

During this flow, the chamber pressure is controlled to a preferredvalue of approximately 100 mT by using feedback control of a step motorcontrolling a vacuum valve to a pressure set by the processor (step233). Alternately, pressures between 50 mT and 90 T or more preferablybetween 50 mT and 10 T may be used, as directed by an instruction in theprogram memory. The feedback control is accomplished by controlling aservo valve on the vacuum system so that the gas being introduced isevacuated at a rate which provides the desired pressure.

After approximately 10 seconds, the flow of NF₃ is stopped (step 235).Alternately, the flow of NF₃ may be maintained for anywhere from 2-150seconds, as directed by an instruction in the program memory. The nextwafer is then introduced into the chamber (step 237) and metal isdeposited (step 239). Subsequently, the etch process is performed (step241).

Alternately, other fluorinated or chlorinated gases such as ClF₃, C₂F₆,CF₄, Cl₂, etc. may be used. In addition, the wafer may be etched in thesame chamber or a different chamber and the NF₃ gas may be injectedeither with the wafer in the chamber or with the wafer outside of thechamber. In some process sequences, it is desireable to inject thehalogen-containing gas after the layer of metal has been deposited. Thisallows the injection of a passivating gas to be included in thedeposition sequence, eliminating the need for an independent gas flowcontrol sequence, and provides gas remnants in the chamber to passivatethe next wafer to have a metal layer deposited in that chamber. Thewafers may be transferred to another chamber for the etchback step.

It is believed that the exposure of the wafer to a halogen speciesbefore depositing a tungsten layer prevents residue from remaining afterthe subsequent etching step. Although the exact mechanism is notcompletely understood, it is believed that the halogen species mayadsorb on the surface over certain sites where tungsten preferentiallygrows. Thus, the adsorbed halogen passivates those sites and allowscomplete removal of the tungsten during the subsequent etch process.

The NF₃ residues which have been absorbed on the wafer surface thusapparently prevent out-of-phase growth of tungsten on that particularwafer being processed. Thus, the NF₃ injection step of the presentinvention will prevent residue from affecting a subsequently introducedwafer into the chamber.

As an alternative to exposing a wafer to remnants of halogenated gas ina chamber, a halogenated gas was flown directly over the surface of thewafer to passivate the areas of the wafer that might otherwise growmetal that would not be removed in a subsequent etch step. For example,NF₃ was flown onto the surface of the wafer for a period of time priorto the onset of the deposition step. The NF₃ was flown into the chamberfrom the gas head over the surface of the wafer at a flow rate of 30-150sccm. The chamber pressure was 50 mT to 10 T and the flow was maintainedfor 5-30 seconds, before the flow of the passivating gas was shut offand wafer processing continued.

In another example, WF₆ was flown over the surface of a wafer prior to adeposition step to passivate the surface of the wafer. In this instancethe WF₆ was used as a fluorine source, rather than a source of tungsten.In the deposition step, a reducing agent, such as silane or hydrogengas, was added to the chamber to initiate deposition of tungsten.

FIG. 2D is a flowchart of a wafer passivating process where thepassivating gas is flown directly onto the wafer prior to metaldeposition. A wafer is loaded into the chamber (step 243) and apassivating gas is flown over the surface of the wafer (step 245) fromthe gas discharge head. If a different passivating gas, such as NF₃,were used, it may be appropriate to stop the flow of the passivating gasaltogether. Furthermore, if the same gas is used to passivate thesurface of the wafer and to deposit the metal layer, it may be possibleto maintain the same gas flow. Hence, the step of changing the flow ofthe passivation gas (step 247) is optional. After the metal layer wasdeposited, the wafer was removed from the chamber (step 251).Subsequently, the wafer was etched back. The metal residue remainingafter the etch process was less than the metal residue typically seen onan unpassivated wafer.

While it has been recognized that WF₆ may be used as a tungsten sourceto nucleate sites where it is desirable to deposit tungsten, in theprocess illustrated by the flowchart of FIG. 2D WF₆ is being used as afluorine source to passivate the surface of the wafer to reducesubsequent residue after deposition and etch steps. The deposition stepthat follows the wafer surface passivation step typically deposits alayer of tungsten over the entire surface of the wafer. Surfacepassivation results in fewer shorting interconnections and other defectsarising from residual tungsten when that layer of tungsten is patternedin an etch step.

Alternate embodiments of the invention are also possible. For instance,instead of the halogen gas flow being injected before every wafer, theprocessor could control the gas supply and mixing system to inject thehalogen gas before every second, third, fourth, fifth or other number ofwafers (or before such numbers of wafer). In addition to the NF₃ flow ofthe present invention, a plasma clean operation can still be performedunder control of the processor after a number of wafers have beenprocessed, for instance, 25. Another embodiment of combined passivationtechniques, such as overetching a chamber during a cleaning process anddirectly or indirectly exposing a process wafer to passivating gas priorto deposition.

As will be understood by those with skill in the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. For example,lamp-heated chambers could be used instead of resistively-heatedchambers for processing the wafers. Accordingly, the above descriptionis intended to be illustrative, but not limiting, of the scope of theinvention which is set forth in the following claims.

What is claimed is:
 1. A substrate processing apparatus comprising: asubstrate processing chamber; a gas distribution system configured tointroduce gases into said substrate processing chamber; a substratetransfer system configured to transfer substrates into and out of saidsubstrate processing chamber; a plasma generation system configured toform a plasma within said substrate processing chamber; a controlleradapted to control said gas distribution system, said plasma generationsystem and said substrate transfer system; and a memory, operativelycoupled to said controller, that stores a computer-readable program,said computer readable program including computer instructions that: (i)control said substrate transfer system to transfer a first substrateinto said chamber, control said gas distribution system to flow a metaldeposition gas into said chamber to deposit a metal layer over saidfirst substrate and control said substrate transfer system to transfersaid first substrate out of said chamber; (ii) thereafter, control saidgas distribution system to flow a halogen-containing etchant gas intosaid substrate processing chamber and control said plasma generationsystem to form a plasma from said etchant gas, said etchant gas beingflowed into said chamber and said plasma formed within said chamber fora selected period of time after said chamber is cleaned of deposits inorder to overetch said chamber; and (iii) thereafter, control saidsubstrate transfer system to transfer a second substrate into saidsubstrate processing chamber so that said second substrate is exposed toremnants of said halogen-containing etchant gas and control said gasdistribution system to flow a metal deposition gas into said substrateprocessing chamber to deposit a layer of metal over said substrate. 2.The apparatus of claim 1 wherein said halogen-containing gas comprisesfluorine.
 3. The apparatus of claim 2 wherein said halogen-containingfurther comprises chlorine.
 4. The apparatus of claim 2 wherein saidhalogen containing gas is NF₃.
 5. The apparatus of claim 1 wherein saidselected period of time is between about 30-300 seconds.
 6. Theapparatus of claims 1 further comprising a pressure-control system,controllable by said controller, capable of establishing and maintaininga selected pressure in the chamber, and wherein said program furtherincludes computer instructions for establishing and maintaining apressure within the chamber while said halogen-containing etchant isflowed into said chamber of between about 100 mTorr-2 Torr.
 7. Theapparatus of claim 1 wherein said metal deposition gas comprisestungsten and wherein said layers of metal deposited over said first andsecond substrates are tungsten layers.
 8. A substrate processingapparatus comprising: a substrate processing chamber; a gas distributionsystem configured to introduce gases into said substrate processingchamber; a substrate transfer system configured to transfer substratesinto and out of said substrate processing chamber; a controller adaptedto control said gas distribution system and said substrate transfersystem; and a memory, operatively coupled to said controller, thatstores a computer-readable program, said computer readable programincluding computer instructions that: (i) control said gas distributionsystem to flow a halogen containing gas into said substrate processingchamber and then stop said flow of said halogen-containing source; (ii)thereafter, control said substrate transfer system to transfer asubstrate into said substrate processing chamber so that said wafer isexposed to remnants of said halogen-containing gas; and (iii) controlsaid gas distribution system to flow a metal deposition gas into saidsubstrate processing chamber to deposit a layer of metal over saidsubstrate.
 9. The apparatus of claim 8 wherein said halogen-containinggas comprises fluorine.
 10. The apparatus of claim 9 said halogencontaining gas comprises NF₃.
 11. The apparatus of claim 9 herein saidhalogen-containing gas further comprises chlorine.
 12. The apparatus ofclaim 8 wherein said program controls said gas distribution system toflow said halogen-containing gas into said chamber for between about2-150 seconds before said substrate is transferred into said chamber.13. The apparatus of claim 12 wherein said program controls said gasdistribution system to flow said halogen-containing gas into saidchamber at a rate of about 10-2000 sccm.
 14. The apparatus of claim 8wherein said apparatus further comprises a pressure-control system,controllable by said controller, capable of establishing and maintaininga selected pressure in the chamber, and wherein said program furthercomprises instructions for controlling said pressure control system tomaintain said chamber at a pressure between about 50 millitorr-90 torrwhile said halogen-containing gas is flowed into said chamber.
 15. Theapparatus of claim 8 wherein said metal deposition gas comprisestungsten and said layer of metal is a tungsten layer.
 16. The substrateprocessing apparatus of claim 8 wherein said halogen-containing gas is ahalogen-containing, non-metal containing gas.
 17. The substrateprocessing apparatus of claim 8 wherein said program further includesinstructions to, before said halogen-containing gas is flowed into saidchamber, control said substrate transfer system to transfer a secondsubstrate into said chamber, control said gas distribution system toflow a metal deposition gas into said chamber to deposit a metal layerover said second substrate and control said substrate transfer system totransfer said second substrate out of said chamber.